This invention relates to an imaging optical system such as an objective lens used in a video and audio disc, an optical memory device or the like and a collimator lens, and in particular to an imaging optical system in which the numerical aperture NA is up to the order of 0.55 and aberration correction in the vicinity of the axis has been done.
Some of objective lenses used in optical memory devices and the like comprise three to five spherical lens groups as proposed in Japanese Patent Pablication No. 44209/1977, Japanese Laid-open Patent Application No. 148143/1977, etc. It is desired from the viewpoints of not only imaging performance but also the capability of following the automatic focusing and tracking control at a high speed that objective lenses used for this usage be compact and light in weight, but said objective lenses comprising spherical lens groups are not sufficient in this point. Also, said objective lenses are combination lenses and therefore, in such objective lenses, adjustment of the optic axis and assembly adjustment of the lens spacing are the cause of their high cost. In order to solve such problems, the use of a non-spherical single lens as the objective lens for an optical memory device has heretofore been proposed in Japanese Laid-open Patent Application No. 64714/1982, Japanese Laid-open Patent Application No. 201210/1982, Japanese Laid-open Patent Application No. 17409/1983 and Japanese Laid-open Patent Application No. 68711/1983. However, all of these propositions are concerned with the type in which both surfaces are non-spherical or one surface is spherical and generally require the amount of allowable parallel eccentricity to be as high as the order of 1-2 .mu.m. Accordingly, it has been difficult for the actually manufactured lens to obtain a sufficient performance as per the design value.
It is an object of the present invention to provide an imaging optical system which is easy to manufacture and which has a good imaging performance.
To achieve such object, the imaging optical system according to the present invention comprises a non-spherical single lens having a first surface of a non-spherical shape in which the radius of curvature becomes gradually greater away from the optical axis and a planar second surface, and a parallel flat plate disposed adjacent to the second surface of said non-spherical single lens as required. The above object is achieved by satisfying the following conditions: ##EQU2## where f is the focal length of the non-spherical single lens, d is the on-axis thickness of said single lens, n is the refractive index of said single lens, NA is the numerical aperture of said single lens, r.sub.1 is the radius of curvature of the first surface of said single lens in the vicinity of the optic axis, and t is the thickness of said parallel flat plate.
The parallel flat plate disposed as required may be, for example, the substrate glass of a recording medium used in a device for an optical disc DAD or the like, or the right beam exit window of a semiconductor laser. That is, in the optical disc, the light beam condensed on the recording medium is applied through the substrate glass, and where the light beam from the semiconductor laser is to be collimated by a non-spherical single lens, the light beam passed through the exit window provided for the protection of the semiconductor laser is collimated by the lens. Accordingly, in such a state of use, the imaging optical system is an optical system including a parallel flat plate disposed in addition to a non-spherical single lens. Of course, in a different state of use, the non-spherical single lens may be used singly. In this case, the value of t in the aforemention condition is zero.